Measuring Nano- to Microstructures from Relayed Dynamic Nuclear Polarization NMR

We show how dynamic
nuclear polarization (DNP) NMR can be used
in combination with models for polarization dynamics to determine
the domain sizes in complex materials. By selectively doping a source
component with radicals and leaving the target undoped, we can measure
experimental polarization buildup curves which can be compared with
simulations based on heterogeneous distributions of polarization within
the sample. The variation of the integrated DNP enhancement as a function
of the polarization time is found to be characteristic of the geometry.
We demonstrate the method experimentally on four different systems
where we successfully determine domain sizes between 200 and 20 000
nm, specifically in powdered histidine hydrochloride monohydrate,
pore lengths of mesoporous silica materials, and two domain sizes
in two-component polymer film coatings. Additionally, we find that
even in the apparently homogeneous frozen solutions used as polarization
sources in most DNP experiments, polarization is relayed from protons
near the radicals to the bulk of the solution by spin diffusion, which
explains the experimentally observed buildup times in these samples.